There continues to be interest in disposable articles and the disposal of these articles. Examples of such disposable articles include infant diapers, incontinence briefs, sanitary napkins, tampons, bed liners, bedpans, bandages, food containers, agricultural compost sheets, plastic forks, plastic knives, plastic spoons, plastic plates, foamed cups, plastic trays, garbage bags, cigarette filters, and syringes. Examples of other disposable items include razor blade handles, toothbrush handles, disposable fishing lines, fishing nets, packaging, cups, clamshells, and numerous other items. Such disposable items commonly enter the municipal solid waste stream. Due to inadequate treatment of the solid waste and the durable composition of the items, these items accumulate in landfills.
Moreover, many of these disposable items frequently are discarded and become litter items. The number of available landfills is diminishing, and the cost of solid waste disposal is on the rise.
Although such disposable items are not reusable and do not fit into the plastic recycling stream, it has become widely recognized that such items could in effect be recycled for their carbon content by composting. Composting of disposable items reduces the solid waste volume for landfilling and provides a useful product, namely humus. However, one problem with composting is the availability of compostable materials suitable for entry into such a recycling stream.
Although cellulose esters, such as cellulose acetate, are often considered to have a distinct or regular chemical structure, those skilled in the art recognize that cellulose esters are in fact copolymers whose physical properties dramatically depend upon the monomer composition of the copolymer (Buchanan, et al., Macromolecules, 1991, 24, 3050; Ibid., 3060; "Polymer Blends, Vol. 1", D. R. Paul and S. Newman, Academic Press, 1978.). For example, cellulose acetate is in fact a copolymer which can contain up to eight monomers (FIG. 1) while a cellulose mixed ester such as cellulose acetate propionate, is a copolymer which can contain up to 27 monomers.
Miscible polymer blends, often called alloys, are homogeneous mixtures of two or more polymers. Miscible polymer blends typically are optically clear resins having a single composition-dependent glass transition temperature. Miscible blends can be viewed as a subset of compatible blends which results when two materials are combined to give a resin with beneficial properties.
Blends of two or more cellulose esters are generally unknown and believed to be incompatible with little useful properties.
Known examples of blends of two different cellulose esters having a similar degree of substitution but with different types of substituents comes principally from the work of Petersen, et al. (Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 1969, 10, 385) who reported that cellulose acetate (CA)/cellulose acetate propionate (CAP), CA/cellulose acetate butyrate (CAB), and CAP/CAB blends are incompatible (i.e., phase separated in solution or gave a film that was not clear). Kashdan U.S. Pat. No. 4,795,641 discloses that immiscible blends of cellulose acetate and cellulose mixed esters for the controlled release delivery of bioactive agents.
Known examples of blends of two different cellulose esters having similar substituents but having two different degrees of substitution (DS, "DS" refers to the number of substituents per anhydroglucose unit where the maximum DS is three) are equally limited. Slonimskii (J. Polym. Sci., 1958, 30, 625) discloses that blends of cellulose acetate I (DS ca. 3) and cellulose acetate II (DS ca. 2.5) are incompatible (i.e., phase separated in solution). Friese in unpublished work ("Polymer Blends, Vol. 1", D. R. Paul and S. Newman, Academic Press, 1978) suggests that films prepared from cellulose triacetate (DS=3) and cellulose diacetate (DS ca. 2.5) are compatible when cast from ethylene chlorohydrin solutions but incompatible when cast from 1/1 CHCl.sub.3 /acetone. Therefore, this previous work suggests that the films cast from ethylene chlorohydrin still retain solvent.
Some miscible blends of a cellulose ester and another polymer are known in the art. U.S. Pat. No. 3,781,381 discloses that blends, formed by solution casting of polycaprolactone and a cellulose ester mixture, are miscible. Hubbell and Cooper in J. Appl. Polym. Sci., 1977, 21, 3035, demonstrated that cellulose acetate butyrate/polycaprolactone blends are in fact immiscible.
U.S. Pat. No. 3,668,157 discloses that block copolymers of polyether-polyesters form miscible blends with some cellulose esters. The use of an elastomeric block copolymer was reported to be critical. U.S. Pat. No. 3,668,157 also discloses that the corresponding homopolymeric elastomers were incompatible with cellulose esters.
U.S. Pat. No. 4,506,045 discloses that polyester-carbonates and polyether-carbonates copolymers form miscible blends with many cellulose esters and are useful as thermoplastic resins.
U.S. Pat. No. 4,533,397 discloses that contact lenses can be prepared from blends consisting of (A) 97-70% by weight of cellulose esters and (B) 3-30% by weight of an aliphatic polymeric compound having ester moieties, carbonate moieties, or both ester and carbonate moieties in the same polymer chain. The disclosure in U.S. Pat. No. 4,533,397 is limited to aliphatic polymeric compounds; no reference is made to random copolymers consisting of aliphatic diacids, aromatic diacids, and suitable diols or polyols. The disclosure in U.S. Pat. No. 4,533,397 is further limited to cellulose mixed esters having a weight per cent hydroxyl of 1.2% to 1.95% (DS.sub.OH =0.11-0.19). It is also limited to binary miscible blends and to the composition range of the blends (3-30 weight % aliphatic polymeric compound).
The disclosures of Scandola (Macromolecules, 25, p. 6441 (1992)) and Buchanan (Macromolecules, 25, p. 7373 (1992)) concerns miscible blends of cellulose esters with poly(hydroxybutyrate) or poly(hydroxy butyrate-co-valerate). Immiscible blends of cellulose esters and aromatic polyesters have also been disclosed in U.S. Pat. No. 4,770,931 which are useful in applications such as paper substitutes.